1. Field of the Invention
This invention relates to inspection of articles related to the manufacture of semiconductor devices. More particularly, this invention relates to the inspection of photomasks or reticles used in the photolithographic manufacture of semiconductor devices.
2. Description of the Related Art
Modern microelectronic devices are commonly produced using a photolithographic process. In this process, a semiconductor wafer is first coated with a layer of photoresist. In one important technique of production, masks or reticles are used to transfer circuitry patterns to semiconductor wafers. Although there are differences in usage of the terms “mask” and “reticle,” for the purposes of the present invention the terms are interchangeable, and references hereinbelow to either of these terms should be understood as including both masks and reticles, unless otherwise specified. Typically, the reticles are in the form of patterned chrome over a transparent substrate. A series of such reticles are employed to project the patterns onto the wafer in a preset sequence. Each photolithographic reticle includes an intricate set of geometric patterns corresponding to the circuit components to be integrated onto the wafer. The transfer of the reticle pattern onto the photoresist layer is performed by an optical exposure tool such as a scanner or a stepper, which directs light or other radiation through the reticle to expose the photoresist. The photoresist is thereafter developed to form a photoresist mask, and underlying polysilicon insulation or a metal layer is selectively etched in accordance with the mask to form features such as lines or gates.
It should be appreciated by those skilled in the art that to produce an operational microelectronic circuit, a mask must be as defect-free as possible, preferably completely defect-free. Therefore, mask inspection tools are needed to detect various defects in the masks that can potentially reduce the microelectronic circuit fabrication yields. Smaller feature sizes of the masks used in the microphotolithographic process, as well as the use of phase shift and OPC masks, require more sophisticated tools for mask inspection. For instance, the inspection of phase shift masks requires not only finding “conventional” defects, such as particles, but also detecting errors in the thickness of various regions of the mask. Numerous systems for mask inspection have been developed in response to the growing demands of the electronic industry.
From the above description, it should be appreciated that any defect on the reticle, such as extra or missing chrome, may transfer onto the fabricated wafer in a repeated manner. Thus, any defect on the reticle would drastically reduce the yield of the fabrication line. Therefore, it is important to inspect the reticles carefully, and detect any defects thereupon. The inspection is generally performed by an optical system, using transmitted, reflected, or both types of illuminations. An example of such a system is the ARIS21i reticle inspection system available from Applied Materials, Inc., 2821 Scott Boulevard, Santa Clara, Calif. 95050.
There are several known algorithmic methods for inspection of reticles. These methods include: die-to-die inspection, in which a die is compared to a purportedly identical die on the same reticle; and “die-to-database” inspection, in which data pertaining to a given die is compared to information in a database, which could be the one from which the reticle was generated. In another inspection method, die-to-golden-die, a reference die is chosen for inspecting wafers. There also is a design rule based inspection, in which the die has to fulfill line width and spacing requirements, and feature shapes should fit predefined shapes. Examples of these inspection methods, and relevant apparatus and circuitry for implementing these methods, are described in various U.S. patents, including, inter alia, U.S. Pat. Nos. 4,805,123, 4,926,489, 5,619,429, and 5,864,394. The disclosures of these patents are incorporated herein by reference. A die-to-database inspection system is available as the model ARIS100i from Applied Materials, Inc.
Known inspection techniques typically image the article under inspection using a large magnification onto a solid state imaging device, such as a charge-coupled device (CCD) camera. The imaging technique requires the article to be illuminated. The brightness of the illuminating source is a key factor in the ability to speed the inspection by reducing the integration time of the camera. As the patterns on wafers become smaller, it becomes necessary to use smaller wavelengths in order to be able to detect the patterns. This is due to the fact that the physical resolution limit depends linearly on the illumination wavelength, and further due to interference effects, which require that the inspection be done at a wavelength similar to the one used in the lithographic process. As the wavelengths become smaller, conventional incoherent light sources like filament lamps or gas discharge lamps do not have enough brightness, and the light sources of choice become short wavelength lasers. The coherence of the laser, roughness and aberrations of the optical surfaces used in the inspection system, and patterns on the article (such as circuit patterns on a mask, reticle or semiconductor wafer) along the light path combine to create artifacts due to interference and diffraction of the laser beam.
Inspection systems used to detect manufacturing defects can be classified by two interdependent factors: detection rate and false alarm rate (FA rate), referred to herein as false positive detection. Optical inspection systems using CCD/CMOS cameras can suffer from contamination or scratches on the optical surfaces and from detector problems (blemishes, dead pixels, etc.) Such defects can cause artifact images at the CCD plane of the inspection system, especially under high coherence illumination, which may hide actual defects on the article under inspection. The artifacts dynamically change, depending on the article pattern, since the pattern on the article influences the diffraction pattern generated at the CCD plane by a contaminant particle or scratch. The article patterns thus affect the FA rate, an effect that can not be neutralized by calibration procedures. For efficient inspection, there is a need to maintain a low FA rate, while still providing high inspection throughput.